1. Field of the Invention
This invention relates to apparatus used for measuring and testing, and more particularly, it relates to measuring and testing systems used in connection with the drilling of oil wells.
2. Description of Prior Art
Rotary drilling for making a well bore into the earth for the production of oil and gas materials has been practiced since the early twentieth century and its many advantages have been appreciated. Although the components of the well drill string appear to be massive and of great strength when viewed at the earths surface, the well drill string in even a moderately deep well, is in reality a highly flexible and relatively easy to damage drilling tool. Controlling the drill string weight applied to the drill bit is most important for drilling efficiency.
Also, the derrick appears to be massive and of exceptional strength and this is true in all properly designed drilling rigs. For example, the derrick may support a load of 250 tons. An important part of the rig is wire rope or steel cable or wireline that is used to moveably suspend the drill string from the derrick. The cable at one end is wound on a drum or windless and is then passed in several turns between the crown block and traveling block. The other cable end is secured at the derrick foundation or base. The traveling block carries a hook with rotary connection to the kelly. The hook supports the weight of the drill string within the well bore and applied to the drill bit. The cable is usually multi strand with multi wires per strand. For example, a cable or wireline of 11/8 inch diameter can have fiber core surrounded by eight strands each having 19 steel wires. This cable has a breaking strength of about 40 tons but yet is pliable and wear resistant. Although these cables are of great strength, they do have some finite load limit where a breaking problem can occur.
The tension conditions on the wireline in virtually every rotary drilling rig is monitored during present day drilling operations. Various tension measuring systems are employed in the functions of automatic drillers and weight indicators so that the driller has an indication of tension conditions on the wireline and also to control automatically the drill string weight upon the drill bit. One commercial system is shown in U.S. Pat. No. 3,031,169 to A. I. Robinson et al., which issued Apr. 24, 1962 and entitled "Apparatus for Automatically Controlling Drilling". In this system a sensor device is bolted to the wireline and produces a pneumatic signal to a meter on the derrick floor so the driller had some indication of tension conditions on the wireline, especially changes in tension. It could also automatically maintain a selected weight-on-bit loading during drilling.
The sensor device or strain gage was disc shaped and engaged the wireline in a three point attachment between a central plunger and equal distant end supports. The wireline was deflected at small equal angles (.theta.) into the two sides of an isosceles triangle whose base was a straight line between the supports. The wireline tension (T) acted on the plunger with a force (F) according to the formula T=F/2 sine .theta.. The force (F) on the plunger controlled a transmitter that generated the pneumatic signal to the remote, floor mounted receiver. For accuracy the end supports, usually bolted to the wireline, must let it slide a small longitudinal distance during the tension condition measurement.
One improved sensor device used rollers on the end supports for engaging the wireline but yet letting it move a small longitudinal distance in making these measurements. The roller supports were threadedly mounted directly onto the disc. A threaded stop controlled the lateral displacement of the wireline against the plunger.
Another problem with tension measuring system resides in the stiffness of wirelines (e.g., 1 inch or larger in diameter). These lines do not bend readily at the end supports and central plunger of the sensor device to form an accurate isosceles triangle shape. Rather, the stiff wirelines tend to form merging curves and not precise angles at the end supports and central plunger. As a result, there is some magnitude of the Force (F) acting on the central plunger even under zero wireline tension conditions. For this reason, prior tension measuring systems always had an indicated weight readout of some magnitude even under absolute zero tension conditions on the wireline.
These prior sensor devices also all have had a major problem associated with mounting the device on the wireline. It will be appreciated that a well worker or roughneck usually mounts the sensor device on the wireline. This installation is below the derrick floor where the end of the wireline is anchored to the derrick foundation. No more difficult work area can be found. This work area is cluttered with piping, valves, hoses, etc. and everything is coated with either mud, water or oil or all three materials. If a nut, part or tool is dropped, it just disappears. As a result, the well worker has had great problems mounting the sensor device onto the wireline at its anchored end. The many loose pieces, especially nuts and bolts invite one or more pieces to fall and be lost. Usually, a replacement piece cannot be found and this results in the supplier of the device making a trip (usually at no charge) to the well site to supply the lost parts.
Also, the wirelines vary in sizes and characteristics and required the well workers to repeatedly adjust, change, and then move, cut and try to adjust the sensor device to give proper and useful readouts of tension conditions on the wireline and indicated weight on the wireline. These precise adjustments are most difficult because of the location of the sensor device.
The present invention is a tension gage that is readily installed on the wireline adjacent its anchor end, usually by one person, and it has no loose parts that can be dropped or otherwise lost. The installation automatically insures the proper adjustment to provide proper and useful readouts of tension conditions on the wireline. The tension gage is readily and easily adjusted where different wireline characteristics are encountered. Adjustment is provided for easily adjusting the gage to zero wireline tension for a direct readout of zero indicated weight on the wireline.